The present invention relates to a needle thread tension control device for a sewing machine, and particularly to a device comprising a thread supply stopping means for stopping supply of a needle thread at a predetermined timing and a tension adjusting means for imparting the needle thread drawn out from a thread supply source a weaker tensile force equal to or lower than that of a bobbin thread.
In a conventional tension control device, there was provided a thread adjusting disc on a needle thread supply path from a thread supply spool to a thread guide of a take-up lever in order to adjust the tension of the needle thread by regulating the resilient force of a spring mounted on the thread adjusting disc to comply with the thickness of a work fabric to be sewn, the thickness of the thread used, the length of a seam, stitch work, etc.
However, in the conventional tension adjusting device, the spring of the thread adjusting disc is required to exert a somewhat stronger force on the needle thread in order to prevent it from being supplied in excess of the required amount for a subsequent stitching when the take-up lever is driven upward. And the spring is adjusted to exert a tension of ten times as much as a tension of the bobbin thread on the needle thread. Accordingly, the needle thread always remains under a strong clamping pressure, and therefore if the sewing machine is put in motion without loading of the fabric to be sewn at the beginning and ending of a sewing operation (this will be referred to as "idling"), the needle thread can not be fed readily by a feed dog and the needle thread tends to go in the reverse direction during upward motion of the take-up lever, thereby causing an excessive supply of the needle thread, which may cause the needle thread to wrap round a looptaker with the result that the sewing machine will become inoperative.
Japanese Patent Publication No. 53-41580 disclosed a thread control mechanism for use in a sewing machine for electrically controlling a tension adjusting disc, wherein the tension adjusting disc actuated by a solenoid is driven not to give any tension to the needle thread with its solenoid deenergized when the needle thread is supplied from a thread supply spool to form a seam, and in a subsequent period of time can serve to clamp the needle thread with the energized solenoid. Thus, even in the event of idling, the needle thread may be fed by the feed dog so smoothly that the needle thread will not twine around the looptaker.
Japanese Patent Publication No. 60-19278 discloses a thread tightening control device for automatically adjusting supply of a needle thread, in which there is provided between a thread supply spool and a take-up lever a needle thread tension device for providing the needle thread with a tension higher than that to be exerted on a bobbin thread, and between the take-up lever and a needle an electromagnetic thread restraining unit for clamping and releasing the needle thread is provided. When thread tightening is carried out by the elevation of the take-up lever to form an intersection between the needle thread and bobbin thread in the center of thickness of a fabric to be sewn, the electromagnetic thread restraining unit is energized to stop tightening of the needle thread, and then in association of the upward movement of the take-up lever, a predetermined amount of the needle thread is discharged through the thread tension device from the thread supply spool.
The U.S. Pat. No. 4,632,048 discloses an upper thread controlling device provided with a thread tension device and a electrical clamping magnet on the thread supply path extending from the thread supply source to the needle. For the first period of time during which the tension in the upper thread is varied due to changes in the rotational speed of the spindle of the sewing machine, a restraining force is applied to the upper thread by the electrical clamping magnet. For the second period of time during which the upper thread is released from the loop controlling tail and stitch tightening is completed, a tension required for stitch tightening is applied to the upper thread by the lower clamping force of the thread tension device.
The thread control mechanism disclosed in the Patent Publication No. 53-41580 has a problem, however, in that since the needle thread is released with no tension exerted thereon when it is supplied from the thread supply spool in association with the feed motion of the fabric, the thread supply spool and the needle thread are subjected to an inertia force, thereby to supplying more needle thread than required with reference to the amount of the feed stroke. Furthermore, it is always difficult to make a fine adjustment of the tension of the needle thread by use of the thread control mechanism. Another problem is that it is impossible to carry out a tacking stitching which requires a weaker thread tension or a gathering stitching which requires a stronger thread tension.
The upper thread controlling device disclosed in the U.S. Pat. No. 4,632,048 has the following problems. In a period of time from a moment after an upper thread is released from the loop controlling tail to a moment after stitch tightening is completed, despite that tightening of the upper thread is not completed, the electromagnet is released, and therefore during tightening of the upper thread, an extra amount of the upper thread may be drawn out from the upper thread supply source and thus a stable thread condition in stitches can not be achieved. Further, the tension exerted on the upper thread during tightening is adjusted to be a much stronger tension in comparison with the tension of the bobbin thread.
U.S. Patent Application Ser. No. 07/189,139 discloses a needle thread tension control system comprising a pre-tension device for exerting a tension on the needle thread and a thread supply control device which checks and releases the needle thread at respective suitable moments.
In this needle thread tension control system, by means of checking or releasing the needle thread at respective suitable moments according to the thickness of the needle thread, the tension of the needle thread can be controlled automatically.
Therefore, in this tension control system, the pre-tension device can be omitted, and also the pre-tension device is intended to be adjusted into an inoperative condition (i.e. zero pre-tension) for the stitching of ordinary seams.
But, the pre-tension device is installed for stitching of particular seams with intersections of the needle thread and the bobbin thread on the upper surface of the work fabric. That is, by exerting rather high pre-tension on the needle thread, intersections are formed on the upper surface.
However, in stitching ordinary seams, in the case where the pre-tension device is adjusted in the inoperative condition, above-mentioned problems are caused.